1. Field of the Invention
The present invention relates to access to a Flash memory, and more particularly, to a method for managing a plurality of blocks of a Flash memory, and to an associated memory device and a controller thereof.
2. Description of the Prior Art
As technologies of Flash memories progress in recent years, many kinds of portable memory devices, such as memory cards respectively complying with SD/MMC, CF, MS, and XD standards, are widely implemented in various applications. Therefore, the control of access to Flash memories in these portable memory devices has become an important issue.
Taking NAND Flash memories as an example, they can mainly be divided into two types, i.e. Single Level Cell (SLC) Flash memories and Multiple Level Cell (MLC) Flash memories. Each transistor that is considered a memory cell in SLC Flash memories only has two charge levels that respectively represent a logical value 0 and a logical value 1. In addition, the storage capability of each transistor that is considered a memory cell in MLC Flash memories can be fully utilized. More specifically, the voltage for driving memory cells in the MLC Flash memories is typically higher than that in the SLC Flash memories, and different voltage levels can be applied to the memory cells in the MLC Flash memories in order to record information of two bits (e.g. binary values 00, 01, 11, or 10) in a transistor that is considered a memory cell. Theoretically, the storage density of the MLC Flash memories may reach twice the storage density of the SLC Flash memories, which is considered good news for NAND Flash memory manufacturers who encountered a bottleneck of NAND Flash technologies.
As MLC Flash memories are cheaper than SLC Flash memories, and are capable of providing higher capacity than SLC Flash memories while the space is limited, MLC Flash memories have been a main stream for implementation of most portable memory devices on the market. According to the related art, it is possible to further increase the storage density of the MLC Flash memories to achieve more than twice the storage density of the SLC Flash memories. However, various problems of the MLC Flash memories have arisen due to their unstable characteristics. Please note that there are some side effects when such a method of increasing the storage density is applied to the MLC Flash memories. For example, the endurance of the MLC Flash memories may become lower, the reading/writing speed may decrease, the probability of the occurrence of reading/writing errors may increase, and some problems may occur when implementing certain kinds of portable memory devices, such as memory cards complying with the SD standards.
More particularly, as the storage density increases, the page count of each block increases correspondingly. As a result, when a portable memory device implemented according to such a method of increasing the storage density is performing a merging operation, the portable memory device can hardly comply with a certain time out specification of the SD standards due to the high page count. Therefore, a novel method is required for enhancing the control of data access of a Flash memory in a portable memory device, in order to guarantee that the portable memory device can operate normally in a situation where the storage density of the Flash memory increases.